The present invention relates to fuel cells and more specifically, to methods and devices which can manage the operational health of fuel cell stacks.
The past few decades have seen an explosion of interest in environmental matters. One consequence of this has been the beginning of a movement away from fossil fuel based energy sources with their attendant effects on pollution. One seemingly viable alternative to such traditional energy sources, especially for automobiles, is the electrochemical fuel cell.
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to produce electric power and reaction products. Such cells can operate using various reactantsxe2x80x94the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol. The oxidant may be substantially pure oxygen or a dilute stream such as air containing oxygen.
One drawback to current fuel cells is the degradation in a cell""s power output over time. Impurities, either from the fuel stream or generated from within the fuel cell as intermediate species during fuel cell reactions, may be adsorbed or deposited on the surface of the anode electrocatalyst. This blocks portions of the electrocatalyst and prevents these portions from inducing the desired electrochemical fuel oxidation reaction. Such impurities are known as electrocatalyst xe2x80x9cpoisonsxe2x80x9d and their effect on electrochemical fuel cells is known as xe2x80x9celectrocatalyst poisoningxe2x80x9d. Such xe2x80x9cpoisoningxe2x80x9d reduces fuel cell performance by reducing the voltage output from the cell for that cell""s current density. The deposit of electrocatalyst poisons may be cumulativexe2x80x94over time, even minute concentrations of poisons in a fuel stream may result in a degree of electrocatalyst poisoning which hampers fuel cell performance.
The sources of such poisons, as mentioned above, are legion. Reformate streams derived from hydrocarbons or oxygenated hydrocarbons typically contain a high concentration of hydrogen fuel but also typically contain electrocatalyst poisons such as carbon monoxide. Because of such a presence, the fuel stream may be pre-treated prior to its direction to the fuel cell. Pre-treatment methods may employ catalytic or other methods to convert carbon monoxide to carbon dioxide. Unfortunately, pre-treatment methods cannot efficiently remove all of the carbon monoxide. Even trace amounts such as 10 ppm can eventually result in electrocatalyst poisoning.
Fuel cell components and other fluid streams in the fuel cell may also be a source of impurities. As an example, fuel cell separator plates are commonly made from graphite. Organic impurities in graphite may leech out and poison the electrocatalyst. Other poisons may be generated by the reaction of substances in the reactant streams with the fuel cell component materials. A further possible source of poison is from intermediate products in the oxidation process. For cells which use complex fuels such as methanol, this is particularly important.
A few methods have been developed which attempt to overcome the electrocatalyst poisoning issue. The anode may be purged with an inert gas such as nitrogen. However, this method involves suspending power generation by the fuel cell. Another approach is that of introducing a xe2x80x9ccleanxe2x80x9d fuel stream containing no carbon dioxide or other poisons to a poisoned fuel cell anode. Where the adsorption is reversible, an equilibrium process results in some rejuvenation of the electrocatalyst. However, such a method is not effective against irreversibly adsorbed poisons. Furthermore, the recovery of the anode electrocatalyst by such an equilibrium process can be very slow, during which time the fuel cell is unable to operate at full capacity.
Yet another approach is to continuously introduce a low concentration of oxygen into the fuel stream upstream of the fuel cell, as disclosed by Gottesfeld in U.S. Pat. No. 4,910,099. Unfortunately, this approach has its own drawbacks, such as parasitic losses from oxygen bleed, undesirable localized exothermic reactions at the anode, and dilution of the fuel stream.
Wilkinson et al in U.S. Pat. No. 6,096,448 discloses periodic fuel starvation of the anode to increase the anode potential. This oxidizes and removes electrocatalyst poisons. Wilkinson describes three methods of accomplishing this fuel starvation: momentary interruption of the fuel supply by closing valves both upstream and downstream of the fuel supply, periodically introducing pulses of fuel free fluid into the fuel supply, and momentarily increasing the electrical load on the cell without increasing the fuel supply. With each of these methods, the anode potential rises because of fuel depletion at the anode. Unfortunately, none of these methods allow direct control of the anode potential. Furthermore, treatment is applied on a stack basis and hence necessarily causes disruption of stack performance.
From the above, there is therefore a need for devices and methods which address the issue of electrocatalyst poisoning while avoiding the problems associated with the efforts described above.
The present invention provides methods and devices which alleviate the effects of electrocatalyst poisoning while avoiding the problems encountered by the prior art. In one aspect, a controller controls a switch bank which can shunt a voltage source in parallel with each fuel cell in a stack. By controlling the voltage of the voltage source, the current through the fuel cell is directly controlled. By increasing the anode potential of the fuel cell through control of the voltage source, poisons deposited on the electrocatalyst are removed, thereby rejuvenating the fuel cell. Each fuel cell in a stack can be treated in turn, causing a reduction of the effects of electrocatalyst poison on stack performance.
In a first aspect the present invention provides a device for removing catalyst poisons in one or more individual fuel cells in a stack while said stack is in use, said stack having plurality of fuel cells coupled to each other in series, said device comprising:
a switch bank coupled to fuel cells in the stack;
a controller controlling said switch bank;
measuring means to measure a voltage, said measuring means being coupled to said switch bank and to the controller; and
a variable voltage source coupled to the switch bank and to the controller, wherein
said switch bank can couple the variable voltage source in parallel with one or more of the plurality of fuel cells in said stack;
said switch bank can couple said measuring means to one or more fuel cells in the stack such that said measuring means can measure the voltage across said one or more fuel cells;
said controller controls a voltage setting of said variable voltage source and said one or more fuel cells in said stack are coupled in parallel with said variable voltage source.
In another aspect the present invention provides a method of reducing catalyst poisons in a stack of fuel cells while said stack is in use, said method comprising:
a) choosing one or more fuel cells for which catalyst poisons are to be reduced;
b) determining a voltage across said one or more fuel cells;
c) setting a voltage setting for a variable voltage source, said voltage setting being different from said voltage across said one or more fuel cells; and
d) momentarily coupling said variable voltage source to be in parallel with said one or more cells.
In a further aspect the present invention provides a device for a stack having multiple fuel cells in the event that at least one fuel cell in said stack is unable to produce its required power output, said device comprising:
a switch bank coupled to the fuel cell stack;
a controller controlling said switch bank;
measuring means to measure a voltage, said measuring means being coupled to said switch bank and to the controller; and
a variable voltage source coupled to the switch bank and to the controller, wherein
said switch bank can couple the variable voltage source in parallel with at least one of the plurality of fuel cells in said stack;
said switch bank can couple said measuring means to one or more fuel cell in the stack such that said measuring means can measure a voltage across said at least one fuel cell;
said controller controls a voltage setting of said variable voltage source and one or more fuel cells in said stack are coupled in parallel with said variable voltage source.